Optical materials are used for various uses such as a lens, a film, an optical fiber, a light guide plate for liquid crystal display, and a waveguide. Among them, optical polymer materials play an important role as raw materials supporting the basis of the electronics field and the communication technology such as information display, information storage, and information transmission. Therefore, physical properties such as durability and scratch resistance are required at a high level as well as optical properties such as a high transparency, a high refractive index, and fine control of refractive index.
As a method of increasing the refractive index in an optical polymer material, a method of introducing a substituent such as a halogen, elemental sulfur, and an aromatic group into the polymer is known. However, the method of introducing a halogen has problems such as decrease in transparency due to the coloration of the optical material, and high environmental load. The method of introducing elemental sulfur has problems such as generation of odor derived from sulfur and decrease in stability due to disulfides by-produced from impurities. Furthermore, the method of introducing an aromatic group has problems such as increase in chromatic aberration due to decrease in Abbe number.
As a method of increasing the refractive index in an optical polymer, a method of combining inorganic particles of titanium oxide, zirconium oxide, or the like with an organic constituent is attracting attention. This method does not have such disadvantages as in the above method of introducing a substituent into the polymer, but has a problem of decrease in transparency caused by the agglomeration of the inorganic particles. In order to solve this problem, techniques to disperse the particles have been studied actively.
For example, a method in which a phosphate compound having a polyoxyethylene alkyl structure is used as a dispersing agent for zirconia particles (see, e.g. Patent Literature 1) and a method in which a phosphate compound having an aromatic group is used as a dispersing agent for zirconia particles (see, e.g. Patent Literature 2) are known. It is reported that the transparency of the polymer is improved by the improvement of the dispersibility of the zirconia particles in these methods. However, the transparency is still not sufficient, and further improvement in optical performance has been demanded. Moreover, since the scratch resistance (the hardness of the polymer) is decreased, further improvement in physical performance is also required.